Lookup for "450 4.0.0"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (450)
Requested mail action not taken: mailbox unavailable [E.g., mailbox busy]
Enhanced Status Code Details (4.0.0)
Other undefined Status
Other undefined status is the only undefined error code. It should be used for all errors for which only the class of the error is known.